An LED lamp, which has a longer lifespan and consumes less power than a fluorescent lamp, has been proposed as a replacement for a fluorescent lamp to achieve reductions in power consumption and a frequency with which the lamp is replaced at the end of its life (for example, Japanese Patent Application Publication Nos. 2008-103304, 2009-266432, and 2008-282793). For example, a straight tube LED lamp serving as a replacement for a straight tube fluorescent lamp includes an elongated substrate having a length that corresponds to a fluorescent lamp, and a plurality of LEDs arranged on the substrate in its lengthwise direction. An aluminum heat sink constituting a part of an outer shell is attached to an opposite side of the substrate from the LEDs, and an increase in a temperature of the LEDs is suppressed by a heat radiation effect of the heat sink, thereby preventing a reduction in a light emission efficiency of the LEDs. Further, a lighting circuit including an AC/DC converter is built into the LED lamp, and an alternating current voltage input from a lighting fixture is converted into a direct current voltage through the lighting circuit, whereupon the direct current voltage is supplied to the LEDs in order to light the LEDs.
In this type of alternating current-lit LED lamp, the lighting circuit is built into the LED lamp, and therefore a lifespan of the lighting circuit is likely to be shortened due to heat generated by the LEDs. As a result, the lifespan of the LED lamp itself is also likely to be shortened. Further, the temperature of the LEDs may be raised by heat generated in the lighting circuit, leading to a reduction in the light emission efficiency of the LEDs. Moreover, when the lighting circuit is inbuilt, a size and a cost of the LED lamp increase. Hence, advancements have been made in the development of a direct current-lit LED lamp that is lit by providing a lighting circuit including an AC/DC converter in a lighting fixture or the like on the exterior of the lamp, for example, and supplying a direct current voltage to a cap of the LED lamp.
Incidentally, the heat sink is made of aluminum and is therefore conductive. In a condition where the LED lamp is attached to the lighting fixture, however, the heat sink is not grounded. Therefore, when a current leaks to the heat sink from an LED module or the lighting fixture, an electric shock may occur upon touching the heat sink. In a direct current-lit LED lamp in particular, a voltage to ground of the heat sink is a simple sum of a power supply voltage and a lamp voltage, which is larger than an effective value of a voltage to ground of an alternating current-lit LED lamp, and therefore the risk of an electric shock is high.